Published US application number 2004/0048600, assigned to Hitachi Ltd., discloses a power saving management for a navigational device. The system according to said application discloses a system comprising a mobile device with navigation software and a receiver for receiving location information from Global Positioning System (GPS). Furthermore, the mobile device has telecommunication means for contacting the navigation server for downloading navigation information. The user of the disclosed system inputs the desired destination to the mobile device. As on response the system requests the current location coordinates of the mobile device from the GPS receiver and sends the coordinates and the destination to the navigation server. The navigation server calculates the route from the current location to the destination and sends the route information back to the mobile device. The route information is constructed from the map information comprising a road network. The road network is modeled as a weighted graph that includes further information depending on traffic rules, for example, prohibited directions. Besides the weight that typically is a classification for a road or street, the edges of the graph have a length that equals to the length of a road or street segment between two junctions.
A common problem with the implementations as described above is that mobile devices do have restricted time of operation because of high power consumption and low power capacity. The application mentioned above suggests a solution wherein the display device of mobile device is turned off when there is no need for further navigation information. For example, if further information is required only after 15 minutes of driving, the display device can be turned off until the next instructions. This allows major power saving as the display device is typically the most power demanding unit in a mobile device.
The problem with the above mentioned solution is that when the display is turned off, the user does not have any information about the route. The user does not know the distance to the next turn nor has any other information about the route. This can be a major drawback if the navigation system does not have all the information of the route and the user has to cope with this by making the right decisions. This is common, for example, in the case of road construction. Furthermore, usually the drivers are willing to consult a map and obtain other information about the route.